The present invention relates to a shift control device for an automatic transmission, and particularly to a technique for transient downshift control executed when a value of transmission input torque becomes reduced to below a torque value needed to advance a shift during downshifting with an accelerator pedal depressed (with a throttle whose opening increases) or in a power-on condition of the vehicle.
As is well known, automatic transmissions in automotive vehicles operate to determine a desired power flow or a power-transmission path in a power train, that is, a desired transmission range gear mode (a desired gear position) by selectively hydraulically operating friction elements such as a plurality of clutches and brake bands, and to perform shifting from a range gear mode to another range gear mode by properly switching engagement/disengagement of each of the friction elements. In determining the desired range gear mode matching to engine/vehicle operating conditions, the automatic transmission uses a throttle opening (regarded as engine load) and a vehicle speed. A shift pattern is preprogrammed, so that a lower gear is selected as the throttle opening (engine load) increases. Therefore, when the driver depresses or pushes the accelerator pedal (or throttle pedal), downshifting to a lower gear occurs. Such downshifting with the accelerator pedal depressed will be hereinafter referred to as a xe2x80x9cdrive downshiftxe2x80x9d or a xe2x80x9cpower-on downshiftxe2x80x9d. Conversely, when releasing the accelerator pedal from the depressed state, upshifting to a higher gear occurs. Such upshifting with the accelerator pedal released will be hereinafter referred to as a xe2x80x9ccoast upshiftxe2x80x9d. Usually, the previously-noted drive downshift is achieved as follows.
At the former stage of the drive downshift, the transmission input speed begins to rise by virtue of the transmission input torque owing to a drop in working-fluid pressure applied to each of friction elements to be disengaged or released (that is, owing to a drop in working-fluid pressure on the released side), and thus each of the friction elements to be disengaged or released begins to slip, with the result that an inertia phase starts. At the latter stage of the drive downshift, an electronic transmission controller determines that the inertia phase has been terminated as soon as the transmission input speed reaches an after-drive-downshift-termination synchronous speed, and then the controller completes the shifting operation by a rise in working-fluid pressure on the applied side). In this manner, a drive downshift is smoothly achieved with less shift shocks. However, if the current engine/vehicle operating conditions (including the current throttle opening) change, for example, in the event that the accelerator pedal is released from the depressed state, engine power output tends to drop. Thus, the transmission input speed itself cannot rise. Under such a condition, there is a tendency for the transmission input speed to decrease. When an effective gear ratio i (=Nt/No) of the transmission input speed (or turbine speed) Nt to transmission output speed No reaches an after-drive-downshift-termination gear ratio, the controller usually determines that the transmission input speed rises up to the previously-noted synchronous speed and thus the inertia phase has been terminated. For the reasons set forth above, in the presence of the release of the accelerator pedal after depression, there is a possibility that the effective gear ratio cannot rise intendedly, due to the undesirable decrease in transmission input speed Nt. In this case, the inertia phase cannot terminate for a long period of time. In other words, the system has difficulty in accurately determining a timing of termination of the inertia phase. This prevents a smooth shifting operation, that is, a smooth drive downshift and a properly timed inertia-phase termination. To avoid this, Japanese Patent Provisional Publication No. 6-129528 (hereinafter is referred to as xe2x80x9cJP6-129528xe2x80x9d) teaches the rise in working-fluid pressure on the applied side at a first predetermined time rate of increase and the drop in working-fluid pressure on the released side at a second predetermined rate of decrease, regardless of the decision result based on the effective gear ratio (Nt/No), for forcibly advancing drive downshift in the presence of the return of the accelerator pedal from the depressed state to undepressed state during the drive downshift. This control will be hereinafter referred to as a xe2x80x9cdrive-downshift forcible termination controlxe2x80x9d. In the system disclosed in JP6-129528, an electronic transmission controller determines that the accelerator pedal is returning from the depressed state to undepressed state, when the transmission input speed tends to reduce during the drive downshift. However, there are some drawbacks, if the controller uniformly determines the presence or absence of the return of the accelerator pedal from the depressed state to undepressed state by way of detection of a drop in the transmission input speed Nt during the drive downshift.
Assuming that a command value of the working-fluid pressure on the released side is reduced according to a preprogrammed time rate of decrease when downshifting occurs due to a stepwise increased throttle opening, there is a possibility of great shift shocks. To avoid this, Japanese Patent Provisional Publication No. 10-47468 (hereinafter is referred to as xe2x80x9cJP10-47468xe2x80x9d) has disclosed a downshift control device that ensures a smooth and moderate rise in the turbine speed Nt by way of a temporary rise in the command value of the working-fluid pressure on the released side during the inertia phase. However, in the presence of negative transmission input torque fluctuations, in the presence of positive fluctuations in a friction coefficient of the friction element to be released, or in the presence of positive fluctuations in working-fluid pressure applied to the friction element to be released, there is an increased tendency the transmission input speed (turbine speed) Nt to reduce during the inertia phase. Under such a condition, suppose the previously-described way (of JP6-129528) to determine the return of the accelerator pedal from the depressed state to undepressed state during the drive downshift is used. In such a case, although the transmission input torque value exceeds a set value but a rate of change in the transmission input speed is below a set value, the controller erroneously determines that a decrease in the rate of change in transmission input speed arises from a drop in the transmission input torque. Thus, the process of shifting down is wastefully advanced forcibly. This may result in shift shocks rather than smooth downshifting. As discussed above, as a first problem, in the system disclosed in JP6-129528, it is difficult to accurately detect or determine a transmission-input-speed drop arising from the return of the accelerator pedal from the depressed state to undepressed state during the drive downshift. As a second problem, when moderately releasing the accelerator pedal such that the transmission input speed (turbine speed Nt) continues to rise slightly, in the system of JP6-129528 it is difficult to detect the return of the accelerator pedal from the depressed state to undepressed state during the drive downshift. In this case, it is impossible to timely initiate the drive-downshift forcible termination control disclosed in JP6-129528. Additionally, when a value of torque created by the turbine runner itself tends to reduce owing to a rise in the turbine speed without returning the accelerator pedal to its undepressed state, the turbine speed has already been risen, but a time rate of increase in the turbine speed is excessively low. In such a case, it is impossible to timely initiate the drive-downshift forcible termination control disclosed in JP6-129528. As a third problem, under a particular condition that there is a great difference between transmission input speeds before and after shifting and additionally a value of transmission input torque is negative, the rotational speed of the engine that causes the negative transmission input torque is risen forcibly, during the drive-downshift forcible termination control according to which the working-fluid pressure on the applied side is risen at the first predetermined time rate of increase and the working-fluid pressure on the released side is dropped at the second predetermined rate of decrease. Thus, a great negative torque acts on the transmission output shaft. This results in pull-in torque shocks, in other words, an undesiredly great vehicle""s deceleration rate (that is, uncomfortable shift feeling). As a fourth problem, the drive-downshift forcible termination control disclosed in JP6-129528 is based on the assumption that during the drive downshift friction elements to be engaged or applied exist. In case of an one-way clutch shifting process in which a friction element to be applied does not exist, the drive-downshift is caused only by releasing the friction element being in its engaged state and thus engaging the one-way clutch. Therefore, the drive-downshift forcible termination control disclosed in JP6-129528 cannot be applied to the drive-downshift based on the one-way clutch shifting process.
Accordingly, it is an object of the invention to provide a shift control device for an automatic transmission, which avoids the aforementioned disadvantages.
It is another object of the invention to provide a shift control device for an automatic transmission, which is capable of distinguishing a transmission-input-speed drop arising from the return of an accelerator pedal from its depressed state to undepressed state during drive downshift from a transmission-input-speed drop arising from the other factors to accurately detect or determine the transmission-input-speed drop based on the return of the accelerator pedal to the undepressed state during drive downshift.
It is a further object of the invention to provide a shift control device for an automatic transmission, which is capable of detecting the presence or absence of moderate releasing action of the accelerator pedal such that the transmission input speed continues to rise slightly, or detecting a reduction in torque created by the turbine runner (an excessively low time rate of increase in the turbine speed), arising from a rise in the turbine speed with no return of the accelerator pedal to its undepressed state so as to timely initiate the drive-downshift forcible termination control.
It is another object of the invention to provide a shift control device for an automatic transmission, in which the drive-downshift forcible termination control (executed due to a lack of transmission input torque) can be applied to a one-way clutch shifting process.
It is another object of the invention to provide a shift control device for an automatic transmission, which is capable of properly terminating the drive-downshift forcible termination control for a shortest possible period of time.
It is a still further object of the invention to provide a shift control device for an automatic transmission in which the drive-downshift forcible termination control is timed not to exert a bad influence upon a next shifting process.
It is another object of the invention to provide a shift control device for an automatic transmission in which, when the drive downshift is a non one-way clutch shifting process, the drive-downshift forcible termination control is executed so that the working-fluid pressure on the applied side is risen at a first predetermined time rate of increase while the working-fluid pressure on the released side is fallen at a second predetermined rate of decrease, and therefore an inertia phase can be terminated for a shortest possible period of time with less shift shocks.
It is another object of the invention to provide a shift control device for an automatic transmission which is capable of providing a desired shift feeling during the drive-downshift forcible termination control by optimally setting the previously-noted first and second predetermined time rates depending upon a quantity of state related to transmission input torque and a quantity of state related to the difference in transmission input speeds before and after shifting.
It is another object of the invention to provide a shift control device for an automatic transmission which is capable of preventing undesired pull-in shocks (that is, undesired vehicle""s deceleration rate), even when the drive downshift is a non one-way clutch shifting process, the vehicle is conditioned in a particular state wherein there is a big difference in transmission input speeds before and after downshifting and a transmission input torque value is negative and thus the pull-in shocks may occur during the drive-downshift forcible termination control, and additionally a next shifting process corresponds to an upshift to a gear position that the transmission has been placed before the downshifting.
In order to accomplish the aforementioned and other objects of the present invention, a shift control device for an automatic transmission capable of automatically shifting to a desired gear position by selectively engaging a plurality of friction elements via working-fluid pressure, and of switching to a drive-downshift forcible termination control when a time rate of change in transmission input speed is reduced to below a threshold value owing to a drop in transmission input torque during drive downshift from a higher gear position to a lower gear position with an accelerator pedal depressed, the shift control device comprises a drive-downshift forcible termination control section that executes the drive-downshift forcible termination control only when the time rate of change in transmission input speed is reduced to below a first predetermined threshold value and a value equivalent to the transmission input torque is reduced to below a second predetermined threshold value, to forcibly terminate the drive downshift.
According to another aspect of the invention, a shift control device for an automatic transmission capable of automatically shifting to a desired gear position by selectively engaging a plurality of friction elements via working-fluid pressure, and of switching to a drive-downshift forcible termination control when a time rate of change in transmission input speed is reduced to below a threshold value owing to a drop in transmission input torque during drive downshift from a higher gear position to a lower gear position with an accelerator pedal depressed, said shift control device comprises a drive-downshift control determining section that determines whether the transmission is in a drive downshift control, an inertia phase control determining section that determines whether the transmission is in an inertia phase control according to which a transmission input speed is rising to an after-shifting synchronous speed, an arithmetic-calculation section that calculates a time rate of change in transmission input speed, a first comparison section that compares the time rate of change in transmission input speed to a first predetermined threshold value, an estimation section that estimates a value equivalent to the transmission input torque, a second comparison section that compares the value equivalent to the transmission input torque to a second predetermined threshold value, and a drive-downshift forcible termination control section that executes the drive-downshift forcible termination control only when the time rate of change in transmission input speed is reduced to below the first predetermined threshold value and the value equivalent to the transmission input torque is reduced to below the second predetermined threshold value, to forcibly terminate the drive downshift. The shift control device may further comprises a first determining section that determines whether the drive downshift is a one-way clutch shift caused by self-engagement of a one-way clutch while releasing a friction element being in its engaged state or a non one-way clutch shift, a second determining section that determines whether there is a possibility of a great vehicle deceleration rate occurring owing to a big difference in transmission input speeds before and after shifting and a great negative engine output torque, a third determining section that determines whether a next shifting process executed after termination of the drive downshift control is an upshift that returns to a gear position established before the drive downshift, and a mode-selection section that selects a first operating mode that forcibly terminates the inertia phase control with a first predetermined time delay when the drive downshift is the non one-way clutch shift without self-engagement of a one-way clutch and there is a less possibility of the great vehicle deceleration rate, and selects a second operating mode that immediately forcibly terminates the drive downshift control and immediately start the next shifting process when the drive downshift is the non one-way clutch shift and there is a possibility of the great vehicle deceleration rate and the next shifting process executed after termination of the drive downshift control is the upshift that returns to the gear position established before the drive downshift, and selects a third operating mode that forcibly terminates the drive downshift control with a second predetermined time delay when the drive downshift is the one-way clutch shift with self-engagement of the one-way clutch. At the first operating mode a working-fluid pressure of a first friction element to be applied is increased at a predetermined time rate of change and a working-fluid pressure of a second friction element to be released is decreased at a predetermined time rate of change, and at the second operating mode the working-fluid pressure of the first friction element is immediately increased up to a predetermined maximum pressure level and the working-fluid pressure of the second friction element is immediately decreased down to a predetermined minimum pressure level, and at the third operating mode only the working-fluid pressure of the second friction element is decreased at a predetermined time rate of change. Preferably, the first predetermined time delay is preferably determined based on the predetermined time rate of change for the first friction element to be applied and the predetermined time rate of change for the second friction element to be released, and the second predetermined time delay is determined based on the predetermined time rate of change for the second friction element to be released. More preferably, the predetermined time rate of change for the first friction element to be applied and the predetermined time rate of change for the second friction element to be released are determined depending upon both a quantity of state related to the transmission input torque and a quantity of state related to a difference in transmission input speeds before and after shifting, so as to provide a desired shift feeling. It is preferable that the predetermined time rate of change is preset to a maximum rate of change of a range within which there is no undershoot of the working-fluid pressure of the second friction element. More preferably, a determination of termination of the drive downshift control is made at a time when the working-fluid pressure of the second friction element is reduced to below a predetermined pressure level, and a next shifting process is inhibited for a period of time from this time.